We’ve all heard the classic advice: 8 glasses of water per day. Or perhaps you’re more inclined to follow the universal bodybuilder’s recommendation of a gallon per day. That should be plenty as long as you don’t dehydrate yourself by drinking a beer or a caffeinated beverage, right? Especially if you’re using the perfect amount of electrolyte packets.
In case I haven’t made my point, there are a lot of myths and dogmatic heuristics surrounding “optimal” hydration practices. As it turns out, these hydration myths don’t hold much water. So, let’s dig into the science behind proper hydration.
Why Hydration Is Worth a Second Thought
Before we put specific claims under the microscope, it’s worth establishing why hydration deserves attention in the first place. As it turns out, dehydration (also known as hypohydration) is far more common than most people assume. A systematic review of soccer players found that a huge fraction of players (roughly half to two-thirds) showed up to training or matches mildly dehydrated. You might expect that the pros are more mindful of their preparedness, but 66.2% of professional soccer players showed up in a dehydrated state. That might sound crazy, but you actually feel pretty normal when you’re only mildly dehydrated, so it often goes unnoticed. But that doesn’t mean there are no consequences for performance.
Only a ~1% reduction in body mass from water loss appears to increase pain sensitivity. In one study, mild hypohydration raised post-exercise pain ratings by about 13% and reduced pain tolerance by roughly 8%. In endurance settings, dehydrated cyclists reported more fatigue and pain than their properly hydrated counterparts. If you push dehydration a bit further to around 3% of body mass, the performance costs increase substantially. Participants in a leg extension study completed about 28% fewer repetitions and reported significantly greater fatigue. For many lifters, inflating your perceived exertion has important practical impacts. If you autoregulate your training based on how hard a set feels, dehydration can distort those metrics and nudge your programming in the wrong direction.
When pushed to the extreme, dehydration concerns get a lot more serious than performance impairment. Acute dehydration dramatically increases the risk of exertional heat illness whereas chronic dehdyration has been linked to negative effects on the cardiovascular, neurological, gastrointestinal, musculoskeletal, and renal (urinary) systems. In summary, the consequences of poor hydration span all the way from from a slightly worse workout to a potentially fatal medical emergency. This spectrum of consequences is summarized in Table 1.
How’d We Get the 8 Glasses Per Day Recommendation?
Let’s start with the most famous hydration heuristic of them all, which is to consume eight, 8-ounce glasses of water per day (which works out to half a gallon). This recommendation is constantly repeated, but it almost never comes with an original citation. As a result, a scientist sought out the hard evidence to support this heuristic, documenting his efforts in a 2002 paper. He didn’t find a bunch of carefully conducted controlled trials, or even a light sprinkling of superficial epidemiological evidence. The best he could do was trace the recommendation back to a comment in the obituary of a nutritionist named Fredrick J. Stare (seriously), along with a passage in a book Stare co-authored suggesting that adults consume somewhere around 6 to 8 glasses of fluid per day.
Notably, the book excerpt is often misinterpreted. According to Valtin, the excerpt specifically states: ““How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. Fruits and vegetables are also good sources of water.” So the 6-8 glasses is not a literal recommendation for glasses of water; it appears to be a recommendation for total fluid intake from all sources (including food and other beverages). It also includes some caffeinated and alcoholic beverages, which people often recommend avoiding due to their alleged “dehydrating effects” (more on this later). So it seems like this recommendation was totally misinterpreted right from the jump, and is often parroted in conjunction with heuristics that contradict the original claim.
The bodybuilder’s common recommendation for GOWAD (gallon of water a day) is even murkier. I’ve never been able to pin down where it came from, but my best guesses are as follows. Bodybuilders have a well-earned reputation for gravitating toward maximal, all-or-nothing approaches, so if regular folks are told to drink half a gallon, a bodybuilder will double it. It’s also worth noting that a one-gallon jug happens to be about the largest fluid container a person can conveniently acquire and lug around all day, so the target may have simply been chosen to match the biggest and most accessible container that’s easy to carry. Over the years, I’ve also found on many occasions that a huge number of bodybuilding traditions and heuristics stem from the use of performance enhancing drugs. So I wouldn’t write off the possibility that extra hydration is favored as a strategy to accommodate the huge list of bodybuilding drugs that can impact fluid balance. Whatever the origin, it’s a heuristic in search of evidence rather than the other way around.
So How Much Do You Actually Need?
If we want a real answer, the best available data come from a 2022 study by Yamada and colleagues, who used doubly labeled water data to examine water turnover in 5,604 people across 26 countries, spanning ages from 8 days to 96 years. Doubly labeled water is both 1) the gold standard for measuring water turnover, and 2) obscenely expensive. So a dataset with thousands of measurements is an absolute goldmine for addressing water needs.
To clarify terminology, water turnover simply refers to the total volume of water moving through your body each day, which represents how much water you need to replace each day. Water leaves your body through several routes and is replaced through several others, including a portion that your body manufactures itself. Those inputs and outputs are laid out in Table 2, along with rough shares for each one.
A key insight from Table 2 is that water turnover is not the same as how much you need to drink. Once you subtract the metabolic water your body makes (roughly 10% of turnover) and those minor respiratory and skin inputs, only about 85% of your turnover has to be replaced through food and beverages. But let’s go to the main finding of the Yamada paper: on average, total turnover is around 4.3 liters per day in adult men and 3.4 liters per day in women. The full dataset is visualized in Figure 1, which also displays how water turnover varies by age.
[Data from Yamada et al (2022). Image courtesy of the MASS Research Review (massresearchreview.com)]
As you can see in Figure 1, average water turnover values are far from a universal recommendation – there is an enormous amount of variation in the data. At the individual level you’ll see daily turnover values below 2 liters or as high as 10 liters, primarily driven by differences in body size, physical activity, athletic status, air temperature, humidity, altitude, and age. The key factors impacting an individual’s total water turnover are summarized in Figure 2.
[Based on Yamada et al (2022). Image courtesy of the MASS Research Review (massresearchreview.com)]
Yamada’s paper provides a few prediction equations to help people estimate their daily fluid needs. To establish some context, consider a couple of examples provided in the paper. A non-athletic 20-year-old man weighing 70 kg, living in a temperate climate, has a predicted turnover of about 3.2 liters per day, while a comparable 60 kg woman comes in around 2.7 liters. A 70 kg athlete training at altitude in hot, humid conditions, and predicted turnover climbs past 7 liters per day. So, how much fluid do YOU need to drink every day?
As we’ve discussed, it depends greatly on your location and lifestyle. But that’s not where the complication ends. Research suggests food supplies anywhere from 20% to 50% of daily water intake, dependong on how you eat. If your diet is built around fruit, raw vegetables, soups, and other minimally processed, water-rich foods, you’ll sit toward the high end of that range. If you live on mostly dry, heavily processed foods that have had the water cooked or stripped out of them, you’ll sit toward the low end. In that case, you’ll need to make up the difference by drinking more fluids.
Rather than posting the (very long and complicated) equations in this article, I’ll refer you to a free tool offered by Built With Science. Their water intake calculator is based on the Yamada equation and can help you start to examine your own water needs based on your characteristics. However, please remember Figure 1 – specifically, the fact that individual needs are subject to very substantial variation. So, predictive tools give you an evidence-based starting point, which should be followed by a process of ongoing hydration tracking practices to help you refine and personalize your needs.
Simple Ways to Track Your Hydration
There are some very simple ways to track and adjust your hydration practices. The first strategy is urine color tracking, which you can visually compare against the guide in Figure 3. A pale straw color is the sweet spot, while dark urine is a cue to drink more. If your urine is completely clear all day long, you might be overdoing it a bit. It’s usually not a huge deal unless you’re taking in fairly extreme amounts of fluid (well over a gallon a day) or underconsuming key electrolytes like sodium and potassium.
[Image courtesy of the MASS Research Review (massresearchreview.com)]
The second strategy is body weight tracking. Weight should tick up slightly throughout the day, as you typically wake up with an empty stomach and slight dehydration. This is why your first urination of the day ought to be your darkest in color. But aside from the gradual increase throughout the day, your weight should stay fairly stable throughout the day and across the week. If you notice a huge weight drop from before your workout to after, you aren’t hydrating enough. The same goes for huge weight drops when you’re at work or out in the sun for a while. The goal should be to avoid large drops by staying on top of your fluid replacement all day long – it’s not good enough to spend 90% of your day dehydrated and chug a bunch of water at night. You also want to keep an eye on day-to-day variation for the same reasons. While daily weight fluctuations are a reality, keep an eye out for signs that you’re losing significant amounts of water due to poor hydration practices.
The third strategy is to monitor your thirst, which is pretty simple. If you’re thirsty, drink some fluids sooner rather than later. But keep in mind that honoring your thirst is not a foolproof way to ensure adequate hydration throughout the day – sometimes thirst kicks in after a meaningful degree of dehydration has occurred.
Does It Matter What You Drink?
We’ve covered how much to drink and when to drink it, but we’ve got one more concept to address. This brings us to specific beverage selection, and to two of the most persistent hydration myths: that beverages containing caffeine or alcohol dehydrate you.
The most direct evaluation of these claims comes from a 2016 study by Maughan and colleagues which developed something called the beverage hydration index. They had adequately hydrated men drink one liter of a test beverage, then collected their urine over the following four hours. They then assessed how well each beverage promoted hydration relative to plain water. Water is assigned a value of 1.0, so any beverage scored above 1.0 hydrates more effectively than water and any beverage scored below 1.0 hydrates less effectively than water (Figure 1).
[Based on Maughan et al (2016). Image courtesy of the MASS Research Review (massresearchreview.com)]
The takeaways are pretty simple. The “dehydrating” impact of caffeine is totally overstated. While caffeine does have a mild diuretic effect (i.e., it increases urine output), this is more than offset by the water content of a coffee or cola. The same is true for a moderate amount of beer – it provides a modest dose of alcohol relative to its water content. However, I should acknowledge that highly concentrated liquor (in isolation) is far worse for hydration, and high volumes of alcohol intake start to inflate the diuretic effect. So 1-2 beers or glasses of wine won’t dehydrate you, but I can’t recommend 10 shots of liquor for hydration purposes.
It’s also worth noting that some common beverages are excellent for hydration purposes. While it’s tempting to assume you need a specifically formulated beverage (like a Gatorade or Pedialyte) to substantially boost hydration, milk and orange juice provide surprisingly similar boosts. Ultimately, your exact beverage choices come down to many factors including specific nutritional needs, preference, tolerability, and context. I don’t expect many NFL teams to replace their sideline Gatorade coolers with giant bottles of milk any time soon – and certainly not in the heat of August.
Summary and Conclusions
When you actually dig into the evidence behind common hydration beliefs and recommendations, many of them lack support. The 8×8 rule traces back to an obituary and a misinterpreted book passage. The gallon jug carried by many bodybuilders is generally plenty, but rests on a shaky foundation of evidence. Coffee doesn’t dehydrate you, and neither does a beer or glass of wine with dinner.
The actual science tells a pretty clear story for hydration. Your fluid needs are individual, driven mostly by your body size, activity level, sweat rate, and climate. Start from a sensible (evidence-based) ballpark estimate, then let your urine color, body weight, and thirst fine-tune the number from there. Focus on staying hydrated all day long, rather than trying to “make up” for morning and afternoon dehydration later in the day. And when it comes to beverage selection, don’t overthink it. Preference and convenience can guide the way – you’re better off selecting something you’re likely to enjoy drinking. At the end of the day, total fluid intake will have a much bigger impact than trying to select the “optimal” hydration beverage.
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